Method for making a lithium ion battery capable of being discharged to zero volts

ABSTRACT

A lithium ion battery particularly configured to be able to discharge to a very low voltage, e.g. zero volts, without causing permanent damage to the battery. More particularly, the battery is configured to define a Zero Volt Crossing Potential (ZCP) which is lower than a Substrate Dissolution Potential (SDP) to thus avoid low voltage substrate damage.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/199,895 filed Apr. 26, 2000.

FIELD OF THE INVENTION

[0002] This invention relates generally to rechargeable batteries andmore particularly to a rechargeable lithium battery capable ofdischarging to zero volts without causing damage to the battery.

BACKGROUND OF THE INVENTION

[0003] Rechargeable lithium batteries are widely discussed in theliterature and are readily commercially available. They typicallyconsist of a positive electrode and a negative electrode spaced by aseparator, an electrolyte, a case, and feedthrough pins respectivelyconnected to the electrodes and extending externally of the case. Eachelectrode is typically formed of a metal substrate that is coated with amixture of an active material, a binder, and a solvent. In a typicalbattery design, the electrodes comprise sheets which are rolledtogether, separated by separator sheets, and then placed in a prismaticcase. Positive and/or negative feed through pins (i.e., terminals) arethen connected to the respective electrodes and the case is sealed.

[0004] The negative electrode is typically formed of a copper substratecarrying graphite as the active material. The positive electrode istypically formed of an aluminum substrate carrying lithium cobaltdioxide as the active material. The electrolyte is most commonly a 1.1mixture of EC:DEC in a 1.0 M salt of LiPF₆. The separator is frequentlya micro porous membrane made of a polyolephine, such as a combination ofpolyethylene and/or polypropylene which can, for example, beapproximately 25 μm thick.

[0005] It is typical to use protection circuitry with lithium ionbatteries to avoid potential deleterious effects. Thus, protectioncircuitry is frequently employed to terminate charging if the voltage ortemperature of the battery (or any cell) exceeds a certain level.Moreover, it is common to incorporate a low voltage cutoff to disconnectthe battery from its load if the voltage of the battery (or any cell)falls below a certain lower level. This latter precaution is taken toprevent permanent damage to the battery which can occur if a voltagegreater than a Damage Potential Threshold (DPT) is applied to one of theelectrodes. For example, corrosion or decomposition of the negativeelectrode substrate can occur if a voltage greater than a SubstrateDissolution Potential (SDP) is applied to the negative electrode.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to a rechargeable lithium ionbattery particularly configured to be able to discharge to a very lowvoltage, e.g. zero volts, without causing permanent damage to thebattery. More particularly, a battery in accordance with the inventionis configured to define a Zero Volt Crossing Potential (ZCP) which islower than the battery's Damage Potential Threshold (DPT) and morespecifically its Substrate Dissolution Potential (SDP), to thus avoidlow voltage substrate damage. The ZCP refers to the voltage of each ofthe electrodes relative to a lithium reference (Li/Li+) when the batterypotential, i.e., the potential between the electrodes, is zero. The SDPrefers to the dissolution potential of the negative electrode substraterelative to the lithium reference (Li/Li+). A conventional lithium ionbattery typically exhibits a ZCP of about 3.6 volts which can slightlyexceed the battery's SDP.

[0007] In accordance with the present invention, the material selectedfor the negative electrode substrate has a dissolution potential greaterthan the ZCP. Commercially pure titanium and titanium alloys arepreferred. Nickel, nickel alloys, and stainless steel can also be used.

[0008] In the normal operation of a lithium ion battery, a solidelectrolyte interface (SEI) layer, i.e., a passivation layer, is formedon the negative electrode, attributable to a reaction between thenegative electrode and the electrolyte. The SEI layer comprises aninsulating membrane that tends to inhibit the continuing reaction of thenegative electrode and electrolyte. It has been recognized that this SEIlayer can dissolve at a voltage above a certain level, i.e., FilmDissolution Potential (FDP), which can lead to permanent damage to thenegative electrode. In accordance with a preferred embodiment of theinvention, the battery is configured to assure a ZCP lower than saidFDP.

[0009] A battery's ZCP level relative to the lithium reference isdependent in part on the materials used for the positive and/or negativeelectrodes. In accordance with a preferred embodiment of the invention,a positive electrode active material, e.g., LiNi_(X)Co_(1-X)O₂ (0<x≦1),is selected which exhibits a discharge curve appropriate to achieve arelatively low ZCP level. This feature of the preferred embodimentfacilitates the implementation of a battery in accordance with theinvention characterized by a Zero Crossing Potential (ZCP) less than itsSubstrate Dissolution Potential (SDP) and/or its Film DissolutionPotential (FDP).

[0010] Batteries in accordance with the present invention areparticularly suited for use in critical applications where physicalaccess to the battery may be difficult. For example, batteries inaccordance with the invention find application in medical devicesconfigured to be implanted under the skin in a patient's body. Such amedical device is typically comprised of a hermetically sealed housingformed of biocompatible material and dimensioned sufficiently small asto be able to be implanted without interfering with normal bodilyfunction. A battery in accordance with the invention includes a caseconfigured for mounting in the device housing. The battery case can beof a variety of shapes, e.g., prismatic or cylindrical, and typicallydefines a volume of between 0.05 cc and 30 cc. Batteries within thisrange exhibit capacities between 1.0 milliamp hours and 3 amp hours. Anexemplary battery for use in such a device includes a prismatichermetically sealed battery casing having dimensions of 35 mm×17 mm×5.5mm. The device is intended to be implanted in the lower back region tohelp alleviate back pain using neurostimulation techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The above and other features and uniqueness of the invention willbe better visualized from the following drawings and schematics.

[0012]FIG. 1A schematically depicts positive and negative batteryelectrodes rolled around a mandrel for placement in a battery case andFIG. 1B depicts in cross-section a complete battery;

[0013]FIG. 2 shows a typical deep discharge curve for a conventionallithium ion battery using copper as the negative electrode substrate andlithium cobalt dioxide LiCoO2 as the positive electrode active material;

[0014]FIG. 3 shows a typical deep discharge curve for a zero voltbattery in accordance with the present invention using titanium as thenegative electrode substrate;

[0015]FIG. 4 shows a typical deep discharge curve for a zero voltbattery in accordance with the present invention usingLiNi_(X)Co_(1-X)O₂ (0<x≦1) as the positive electrode active material;

[0016]FIG. 5 is a table showing preliminary test results of variousbattery configurations in accordance with the present invention; and

[0017]FIG. 6 schematically depicts a battery in accordance with theinvention contained within an implantable medical device housing.

DETAILED DESCRIPTION

[0018] The following description discloses presently contemplatedpreferred embodiments for practicing the invention. This description isnot to be taken in a limited sense, but is offered for the purpose ofdescribing the preferred modes of the invention. The scope of theinvention should be determined with reference to the claims.

[0019]FIGS. 1A and 1B schematically depict a typical lithium ion batteryconstruction 10 comprising a prismatic case 12 containing a positiveelectrode 14 and a negative electrode 16, rolled around a mandrel 18.Separator sheets 20, 22 are incorporated in the rolling to electricallyseparate the electrodes. The case 12 also typically includes electrolytematerial (not shown) and positive and negative feed through pins (i.e.,terminals) 26, 28 which are respectively connected to the electrodes 14,16 and extend externally of the case 12.

[0020] The positive electrode 14 is typically comprised of a thin metalsubstrate, e.g., aluminum, carrying a layer of positive active material,e.g., lithium cobalt dioxide LiCoO₂ mixed with a binder, and coated onboth faces of the substrate. The negative electrode 16 is typicallycomprised of a thin metal substrate, e.g., copper, carrying a layer ofnegative active material, e.g., graphite coated on both faces of thesubstrate.

[0021] Two layers of separator 20, 22 electrically separate theelectrodes 14, 16 from each other, enabling the electrodes to be rolledaround mandrel 18. Each separator layer can comprise a micro porousmembrane made of a combination of polypropylene and is approximately 25μm thick. The electrolyte is most commonly a 1:1 mixture of EC:DEC in a1.0 M salt of LiPF₆.

[0022]FIG. 2 shows typical deep discharge performance curves for aconventional lithium ion battery. The y-axis represents voltage relativeto a lithium reference (Li/Li+) or counter electrode and the x-axisrepresents time. Curves 50 and 52 respectively depict the dischargecurves for the positive and negative electrodes. The battery outputvoltage is the difference between the positive electrode voltage and thenegative electrode voltage. During discharge, the positive electrodevoltage decreases relative to Li/Li+ and the negative voltage increases,primarily near the end of discharge. Typically, a protection ormanagement circuit stops the discharge when the battery voltage reaches2.5 Volts. If the management circuit does not stop the discharge, thenegative electrode potential will rise until it reaches the potential ofthe positive electrode which constitutes the Zero Volt CrossingPotential (ZCP) and is typically about 3.6 volts in conventional lithiumion battery constructions. The negative electrode potential at ZCP,relative to Li/Li+, can exceed the dissolution potential of the negativeelectrode substrate (SDP), e.g., 3.3 volts for copper, and causedissolution and permanent damage to the substrate. The present inventionis directed to battery improvements to assure that the value of SDP isgreater than the value of ZCP, as represented in FIG. 3.

[0023]FIG. 3 depicts deep discharge performance curves for a lithium ionbattery in accordance with the present invention in which the negativeelectrode substrate is formed of titanium instead of copper. The use oftitanium increases the knee of the negative electrode curve 54 toposition the SDP above the ZCP. This relationship considerably reducespotential damage to the negative electrode substrate. In addition tocommercially pure titanium, i.e., titanium CP, other materials can beused to raise the SDP sufficiently, e.g. titanium alloys, nickel, nickelalloys, and stainless steel.

[0024]FIG. 3 demonstrates how the SDP level can be increased relative tothe ZCP by proper choice of the negative electrode substrate material.Alternatively, or additionally, the ZCP level can be decreased relativeto the SDP by proper choice of the positive electrode active material,as depicted in FIG. 4.

[0025] More particularly, FIG. 4 shows the discharge curve 60 for apositive electrode using lithium nickel cobalt dioxide LiNi_(X)Co_(1-X)(where 0<x≦1) as the active material, i.e., as the intercalationcompound. Note that the curve of FIG. 4 exhibits a greater negativeslope than the analogous curve 50 of FIG. 2 representing the standardintercalation compound LiCoO₂ The effect of the increased negative slopeis to lower the ZCP level relative to the lithium reference and the SDP.As was the case in connection with FIG. 3, this reduces the potentialdamage to the negative electrode substrate. Additionally, however, theZCP level also falls below a Film Dissolution Potential (FDP) which isthe voltage above which a solid electrolyte interface (SEI) layer beginsto dissolve. The SEI, or film, comprises a passivation layer which formson the negative electrode and functions to inhibit a continuing reactionbetween the negative electrode active material and the electrolyte.Dissolution of the SEI can noticeably damage the negative electrodeactive material.

[0026] Experiments have been performed at two different temperaturesemploying the aforedescribed techniques depicted in FIGS. 3 and 4. Thepreliminary results are summarized in the table of FIG. 5. Fourdifferent battery configurations were constructed as shown.Configuration (1) corresponds to the conventional arrangementrepresented in FIG. 2 comprising a copper substrate for the negativeelectrode and LiCoO₂ for the positive active material. The battery wasbuilt and then recycled once to get an initial capacity measurement. Thebattery was then shorted between the positive and negative leads toachieve a zero volt state. This zero volt condition was held for oneweek and then recharged and discharged to get a capacity measurementafter zero-volt storage. The capacity retention is calculated bydividing the discharge capacity after zero volt storage by the initialcapacity and multiplying by 100%. In this manner, this percentagereflects any damage that had occurred to the battery while in the zerovolt state.

[0027] As represented in FIG. 5, the capacity retention for batteryconfiguration (1) is below 80%, thus suggesting that damage had beendone to the battery. After opening the battery and examining theelectrodes, it was seen that copper dissolution had occurred. Thisbattery (1) configuration performed poorly at both temperature settings.

[0028] The battery configuration (2) used LiCoO₂ as the positive activematerial and a titanium substrate as the negative substratecorresponding to the arrangement represented in FIG. 3. The results showthat at 25° C. the capacity retention was at about 98% after the zerovolt condition. However, at a higher temperature (37° C.), performancedeteriorates to below 80%. This suggests that perhaps the zero voltcrossing potential was sufficiently below SDP to avoid substratedissolution but still high enough to exceed FDP and cause damage to thenegative electrode active material. Accordingly, attempts were made tolower ZCP further to avoid damage both to the negative active materialand the negative electrode substrate.

[0029] The battery configuration (3) utilizes LiNi_(X)Co_(1-X)O₂ (0<x≦1)as the positive active material with copper negative electrodesubstrate. The results show that at 37° C. the capacity retention isquite high at 90%. However, examination after the test, revealed thatsome copper dissolution occurred. Battery configuration (4) uses bothLiNi_(X)Co_(1-X)O₂ (0<x≦1) as the positive active material and thetitanium as the negative electrode substrate material. Results show thatthis configuration gives the best capacity retention after zero voltstorage.

[0030] From the foregoing table (FIG. 5), it appears that a performancegain is achieved by configuration (2) using a titanium negativeelectrode substrate and by configuration (3) using lithium nickel cobaltdioxide as the positive active material. However, maximum performancegain appears in configuration (4) which combines both of these features.

[0031]FIG. 6 schematically depicts a battery 60 in accordance with theinvention mounted in a housing 64 (shown partially open for the purposesof illustration) of a medical device 66 configured for implanting in apatient's body. The housing 64 is preferably formed of biocompatiblematerial and hermetically sealed. The device 66 is typically used formonitoring and/or affecting body parameters. For example, the device canbe used to electrically stimulate nerves. The casing 68 of battery 64can, for example, have dimensions of 35 mm×17 mm×5.5 mm.

[0032] While the invention has been described with reference to specificexemplary embodiments and applications, it should be recognized thatnumerous modifications and variations will occur to those skilled in theart without departing from the spirit and scope of the invention setforth in the appended claims.

1. A rechargeable battery comprising: a positive electrode; a negativeelectrode said positive electrode comprising a metal substrate having alithium based active material formed thereon; said negative electrodecomprising a metal substrate having a lithium based active materialformed thereon; said positive and negative electrodes defining a ZeroVolt Crossing Potential (ZCP) relative to a reference level when thevoltage between said electrodes is zero; said negative electrodesubstrate being susceptible of permanent damage when a voltage potentialexceeding a Substrate Dissolution Potential (SDP) is applied thereto;and wherein said positive and negative electrodes are configured toestablish ZCP at a lower level than SDP.
 2. The battery of claim 1wherein said negative electrode substrate is formed of a material fromthe group titanium and titanium alloy.
 3. The battery of claim 1 whereinsaid negative electrode substrate is formed of stainless steel.
 4. Thebattery of claim 1 wherein said negative electrode substrate is formedof a material from the group nickel and nickel alloy.
 5. The battery ofclaim 1 wherein said positive electrode active material comprises alithium-nickel-cobalt compound, LiNi_(X)Co_(1-X)O₂ where 0<X≦1.
 6. Thebattery of claim 1 further including an electrolyte; and wherein saidnegative electrode can react to said electrolyte to form a solidelectrolyte interface (SEI) layer, said SEI layer being susceptible ofpermanent damage when a voltage potential exceeding a Film DissolutionPotential (FDP) is applied thereto; and wherein said positive andnegative electrodes are configured to establish ZCP at a lower levelthan FDP.
 7. The battery of claim 1 further including a case for housingsaid positive and negative electrodes; and wherein said case isconfigured for implanting in a patient's body.
 8. The battery of claim 7wherein said case is hermetically sealed.
 9. The battery of claim 7wherein said case has a volume of less then 30 cc.
 10. A rechargeablebattery capable of discharging to zero volts without damaging thebattery, said battery comprising: a positive electrode and a negativeelectrode; at least one of said electrodes being susceptible ofpermanent damage when a voltage exceeding a Damage Potential Threshold(DPT) is applied thereto; said positive and negative electrodes defininga Zero Volt Crossing Potential Threshold when the voltage between saidelectrodes is zero; and wherein said positive and negative electrodesare configured to define a value of ZCP which is less than the value ofDPT.
 11. The battery of claim 10 wherein said negative electrodecomprises a metal substrate having an active material formed thereon,said negative electrode substrate being susceptible of permanent damagewhen a voltage exceeding a Substrate Dissolution Potential (SDP) isapplied thereto; and wherein the value of SDP is greater than said valueof ZCP.
 12. The battery of claim 11 wherein said negative electrodesubstrate is formed of a material from the group titanium and titaniumalloy.
 13. The battery of claim 11 wherein said negative electrodesubstrate is formed of stainless steel.
 14. The battery of claim 11wherein said negative electrode substrate is formed of a material fromthe group nickel and nickel alloy.
 15. The battery of claim 11 whereinsaid positive electrode comprised a metal substrate having a lithiumbased active material formed thereon; and wherein said positiveelectrode active material comprises a lithium-nickel-cobalt compound.16. The battery of claim 11 further including an electrolyte; andwherein said negative electrode can react to said electrolyte to form asolid electrolyte interface (SEI) layer, said SEI layer beingsusceptible of permanent damage when a voltage potential exceeding aFilm Dissolution Potential (FDP) is applied thereto; and wherein saidpositive and negative electrodes are configured to establish ZCP at alower level than FDP.